After drilling a well, such as an oil or gas well, a casing is commonly lowered into the wellbore. Cement slurry is then pumped downhole trough the casing and back up into the annulus between the casing and the borehole. Upon setting, the slurry forms a cement sheath that holds the casing in place, stabilizes and protects the casing. In addition, the cement sheath provides zonal isolation. It is thus critical to prevent upward fluid flow, such as gas migration or water flows, through and along the cement sheath fluid and to prevent exchange between and among formation layers through which the wellbore passes.
Hydraulic cements set and develop compressive strength as a result of hydration of different cement phases. Although this is a continuous process, three main phases can arbitrarily be defined. In the first phase, the cement slurry has a relatively low viscosity and essentially constant Theological properties. This first phase corresponds to the pumping and placement of the cement downhole. In the second phase, the consistency of the cement increases so that it becomes difficult to pump and place it correctly. However, the developed compressive strength is not enough for the cement to be self-supported and to withstand a significant strength. In the third and last phase, the cement continues to develop compressive strength but the well security is insured and the well construction may be resumed.
From a gas migration point of view, it should also be noted that during the first phase, the hydrostatic pressure of the height of cement slurry is high enough to compensate formation gas pressure. Set cement is gas-tight so that if the placement of the cement was adequately performed, no gas migration should occur. However, during the transition phase, gas percolation within the gelling slurry can occur unless the cement slurry is specifically designed and includes for instance gas migration additives such as lattices.
The second phase- or transition phase—is thus not only essentially a waste of time (and consequently of money) but also a period during which the risk of a major accident such as a blow-off is at its maximum. This is why so-called Right-Angle-Set (RAS) cements have received particular attention. RAS cement slurries are defined as well-dispersed systems that show no gelation tendency and set very rapidly due to rapid hydration kinetics; in other words, RAS slurries exhibit a very short transition time.
Due to cement hydration kinetics, designing RAS slurries for temperatures below 120° C. is difficult. The cooler the temperature, the longer the cement set. Therefore, it is especially difficult to design a cement slurry suitable for cold environment such as offshore wells. In deep water wells, temperatures at seabottom are as low as about 4° C., and even lower in arctic zones so that the circulating temperature of the cement typically ranges between 4° C. and 20° C.
This problem of low temperature has been addressed by developing formulations based on specific hydraulic binders. These formulations are essentially divided into two classes: formulations based on gypsum (Plaster) or more exactly on gypsum/Portland cement blend and formulations based on aluminous cement. The performance of aluminous cements might be severely affected by contamination with Portland cements and consequently, they must be stored in separate silos. Therefore, due to logistical reasons, formulations based on plaster have received more attention. Nevertheless, it would be desirable to be able to use more conventional—and less expensive—formulations similar to those used for cementing the deeper part of the well, which are subject to higher temperature due to the thermal geodesic gradient.
According to standard practice, cement slurries are usually formulated with conventional additives including for instance set retarders or set accelerators, dispersing agent and fluid loss control additives. As described, for example, in Well Cementing, edited by E. I. Nelson, Schlumberger Educational Services (1990), an accelerating action has been reported for many inorganic salts including chlorides, carbonates, silicates, aluminates, nitrates, nitrites, sulfates, thiosulfates and alkaline bases such as sodium, potassium and ammonium hydroxydes. Among these, calcium chloride is by far the most common set accelerator used for Portland cement.
However, calcium chloride has several side effects that are not beneficial. Calcium chloride affects the rheological properties of the slurry resulting in an increase of the viscosity. Calcium chloride also has a deterrent impact as to the permeability of the set cement. Lower permeability is not desirable in controlling gas migration and the resistance of the cement to sulfate brines is decreased.
Various other additives or combinations of known additives have been developed for the construction industry, aiming at providing mortars to allow concrete placement at ambient temperature under cold or sub freezing weather. In particular, chloride-free set-accelerating admixtures, based on alkali or alkaline earth metal salt nitrate, nitrite, formate or acetate, have been commercialized. However, it is well known that well cementing exposes Portland cement to conditions far different from those encountered in civil engineering where the specifications with regard to rheological properties, gas migration control and fluid loss control are absent or at least dramatically different.
It is thus an object of the present invention to provide new well cementing formulations that set rapidly at lower temperatures, i.e. below 20° C. and more preferably below 10° C., and that show superior performance over the prior art formulations.